An Obsidian Tag is a string which can be associated with an Obsidian Note.
They can be added inline to the content of the
                    note by adding a hashtag symbol (#)
                    followed by the string, or they can be added to the
                    Obsidian Metadata.
It can only contain the following characters:
- Alphabetical letters
 - Numbers
 - Underscore (
_) - Hyphen (
-) - Forward slash (
/) for Nested tags 
It must also contain at least one non-numerical character.
Source: help.obsidian.md